The rheological properties of fluids are complex and controlled by many parameters. For example, biological fluids, such as blood, are viscoelastic, i.e., they exhibit both viscosity and elasticity.
A number of techniques have been developed or suggested for evaluating the rheological properties of fluids on a microscopic scale. This area of science has become known as microrheology. Several microrheological techniques rely on applying a strain to the fluid through application of an external force, such as a mechanical force or a magnetic field. Such techniques may be undesirable, however, because the fluid under evaluation is evaluated under artificial conditions.
In a more recent approach described in U.S. Pat. No. 6,958,816, microrheological properties of a fluid are observed through low-coherence light scattering. In such analysis, a very small volume of the fluid under evaluation is observed by collecting backscattered rays that are reflected from scattering centers suspended in the volume when light is radiated into the fluid via a single-mode optical fiber. Such an arrangement is depicted in FIG. 1. As is indicated in that figure, light transmitted through the core 100 of a single-mode optical fiber 102 is reflected (arrow A) by scattering centers 104 suspended in a fluid under evaluation 106, and reflected (arrow B) by the end surface 108 of the single-mode optical fiber core. In such an approach, a very small volume 110, e.g., a tenth of a picoliter, of the fluid under evaluation 106 is observed, in part due to the small cross-sectional area of the single-mode optical fiber core 100. Such observation of a very small volume of fluid has been considered preferable given that the received optical signals can become very complex, and therefore difficult to evaluate, when a high concentration of scattering centers are observed due to multiple ray scattering that occurs between the scattering centers.
Although the light scattering approach described above has significant advantages over previous techniques, the signal-to-noise ratio for the received signals is relatively small given that the number of scattering centers from which reflected light is collected is relatively small.